Grazing incidence interferometric devices which can measure the surface contours of a surface of interest by conveying a coherent light beam at grazing incidence to the surface are well-known. FIG. 15 illustrates a typical configuration of these prior art grazing incidence interferometric devices. Such a device conveys collimated, coherent light to a diffractive beam splitter 102 that divides the wave front into two light beams. One of the two light beams, termed the "object beam", is then conveyed at grazing incidence to a surface 2a and the light reflected therefrom is combined with the collimated light (termed the reference beam) which has not been reflected from the surface 2a. Diffractive beam combiner 104 redirects the reference beam so as to be combined with the object beam and travel along a common axis. Imaging lens 106 (in this case the lens of a television camera 108) then forms an interference pattern image of the surface 2a that is recorded by the television camera 108. The surface contours of the surface 2a can then be measured based on the recorded interference pattern image.
However, the lengths of the optical paths from each location on the surface 2a to the interference pattern image formed by imaging lens 106 differ. Thus, there is a problem in that distortions are formed in the interference pattern image recorded by the television camera 108. Hence, the surface contours of surface 2a cannot be accurately measured to as high a precision as would otherwise be possible.
As a partial solution to this problem, a grazing incidence interferometric device as shown in FIG. 16 is known that avoids distortions from being formed in the interference pattern image by positioning an interference pattern observation screen 110 so that its surface lies at the conjugate image of surface 2a. As is apparent from the spacing of the components in FIG. 16, the lens 106 and 112 form an optical system that relays the image of object 2a at unit magnification to interference pattern observation screen 110. As a result of the orientation of the interference pattern observation screen 110 now being conjugate to the object (i.e., the surface 2a to be measured), an interference pattern image that more accurately represents the surface 2a is formed on interference pattern observation screen 110.
The conjugate image arrangement with unit magnification is achieved as shown in FIG. 16, wherein imaging lens 106 is arranged with its focal point at a mid-point of the surface 2a, collimator lens 112 is arranged with its focal point at the image of this mid-point as formed by the imaging lens 106, and the interference pattern observation screen 110 is provided with its mid-point at the focal point of lens 112. Further, the focal distance of the lens 112 is made equal to the focal distance of lens 106, and the interference pattern observation screen is oriented so that its surface is aligned with the conjugate points of the surface 21 as imaged by lens 106 and lens 112.
An alternative prior art arrangement is shown in prior art FIG. 17, wherein a reflecting mirror 114 is arranged at the second focal point of the imaging lens 106 for conveying a bundle of rays of the interference pattern in the reverse direction. A beam splitter 116 is provided between the imaging lens 106 and the diffractive beam combiner 104 to reflect the rays from the reflecting mirror 114 to an interference pattern observation screen 110 that is, once again, provided to have its surface coincide with the conjugate image of unit magnification of the surface 2a. Once again, interference pattern images formed on interference pattern observation screen 110 are viewed by the television camera 108. In this manner the total length of the interferometric device may be prevented from becoming too long.
In the above-described grazing incidence interferometric devices shown in FIGS. 16 and 17, the surface 2a and the observation screen 110 are provided at the conjugate positions where the magnification is 1. In FIG. 16, lens 106 and lens 112 form a lens system, and object 2a and observation screen 110 are positioned with their mid-points a focal length away from this lens system so that unit magnification is achieved. Similarly, in FIG. 17, lens 106 and mirror 114 form an optical system, with object 2a and observation screen 110 again positioned in respective paths with their mid-points positioned a focal length away from the lens system so that unit magnification is achieved. In this way, the television camera 108 records the interference pattern formed by the image of object (surface 2a) and the collimated light from the reference beam, assuming the difference in path length of the object and reference beams does not exceed the coherence length of the light. Thus, instead of using collimator lens 112 as in FIG. 16 to form a unit magnification image of the surface 2a onto observation screen 110, in FIG. 17 the mirror 114 is positioned at the focus of lens 106 to redirect the light backwards through lens 106 to beam splitter 116. The unit magnification image of the surface 2a is thus formed on surface 110, and recorded by television camera 108.
The imaging requirements of such an imaging lens 106 are unique and two-fold. First, the lens 106 must generate only very small aberrations when imaging an object at infinity (i.e., the collimated light of the reference beam). Second, the lens 106 must generate only very small aberrations when imaging surface 2a at unit magnification onto surface 110. Only if both imaging requirements of the lens 106 occur with very small aberrations will the interference pattern observed by television camera 110 accurately enable the surface contours of the surface 2a to be measured accurately. When the collimator lens 112 is used as in FIG. 16, the first requirement mentioned above is nearly satisfied; however, the second requirement mentioned above is not satisfied. As a result, each single point on the surface 2a will not be imaged to a corresponding single point on the screen 110, thus causing problems in that the location of lines in the interference pattern will be imprecise, and the periphery portions of the surface 2a will appear as being out of focus.